This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
In casting thin strip by twin roll caster, the unpredictability of the crown in the casting surfaces of the casting rolls during a casting campaign is a difficulty. The crown of the casting surfaces of the casting rolls determines the thickness profile, i.e., cross-sectional shape, of thin cast strip produced by the twin roll caster. Casting rolls with convex (i.e., positive crown) casting surfaces produced cast strip with a negative (depressed) cross-sectional shape, and casting rolls with concave (i.e., negative crown) casting surfaces produced cast strip with a positive (i.e., raised) cross-sectional shape. The casting rolls generally are formed of copper or copper alloy with internal passages for circulation of cooling water usually coated with chromium or nickel to form the casting surfaces, which undergo substantial thermal deformation with exposure to the molten metal.
In thin strip casting, there is a desired roll crown to produce a desired strip cross-sectional profile under typical casting conditions. It is usual to machine the casting rolls with an initial crown when cold based on the projected crown in the casting surfaces of the casting rolls under typical casting condition. However, the differences between the crown shape of the casting surfaces between cold and casting conditions is difficult to predict. Moreover, the actual crown of the casting surfaces during the casting campaign can vary significantly from that projected crown under typical conditions, since the crown of the casting surfaces of the casting rolls can change even during typical casting due to changes in the temperature of molten metal supplied to the casting pool of the caster, changes in casting speed and other casting conditions, and even with slight changes in the composition of the molten metal as occurs during casting.
Accordingly, there has been a need for a reliable and effective way to directly and closely control the shape of the crown in the casting surfaces of the casting rolls during casting, and in turn, the cross-sectional profile of the thin cast strip produced by the twin roll caster. Previous proposals for casting roll crown control have relied on mechanical devices to physically deform the casting roll, e.g., by the movement of deforming pistons or other elements within the casting roll or by applying bending forces to the support shafts of the casting rolls. Yet, there has not been an effective way to dynamically control the roll crown to produce the desired profile of the cast strip until now.
We have determined that reliable and effective control of the casting roll crown and, in turn, cross-sectional strip profile can be achieved by providing a casting roll of such configuration to enable control of the crown in the casting surfaces by varying casting parameters.
Disclosed is a method of continuously casting thin strip dynamically controlling roll crown comprising the steps of:
a. assembling a caster having a pair of counter rotating casting rolls with a nip there between capable of delivering cast strip downwardly from the nip, where each casting roll has a casting surface formed by a cylindrical tube of a material selected from the group consisting of copper and copper alloy optionally with a coating thereon and having a plurality of longitudinal water flow passages extending through the tube having a thickness of no more than 80 millimeters, the cylindrical tube capable of changing crown of the casting surface with changes in temperature of water flowing through the passages during casting,
b. assembling a metal delivery system capable of forming a casting pool supported on the casting surfaces of the casting rolls above the nip with side dams adjacent ends of the nip to confine the casting pool,
c. positioning at least one sensor capable of sensing thickness profile of the cast strip downstream of the nip and generating electrical signals indicative of the thickness profile of the cast strip,
d. controlling the temperature of the water flowing through the longitudinal water flow passages in the tube thickness,
e. counter rotating the casting rolls and varying the speed of the casting rolls with a casting roll drive system, and
f. controlling the casting roll drive to vary the speed of rotation of the casting rolls and varying the temperature of the water flow circulated through the water flow passages by a control system responsive to electrical signals received from the sensors to control roll crown of the casting rolls during a casting campaign.
The cylindrical tube of each casting roll is of a circumferential thickness that, by varying the casting speed and controlling the temperature of the water circulated through the casting rolls, the crown in the casting surfaces of the casting can reliably be varied to achieve and maintain a desired cross-sectional profile of the cast strip. The thickness of the cylindrical tube may range between 40 and 80 millimeters in thickness or between 60 and 80 millimeters in thickness. The casting rolls may have a cavity internal of the cylindrical tube to define the thickness of the cylindrical tube and facilitate flexure of the cylindrical tube to provide crown control with changes in casting speed and temperature of water circulated through the casting rolls. Water may be circulated through the water flow passages and the cavities of the casting rolls in series. Alternatively, water may be circulated through the water flow passages and then through the cavity of at least one of the casting rolls, or water may be circulated through the cavity and then through the water flow passages of at least one of the casting rolls.
Also disclosed is an apparatus for continuously casting thin strip by dynamically controlling roll crown comprising:
a. a caster having a pair of counter rotating casting rolls with a nip there between capable of delivering cast strip downwardly from the nip where each casting roll has a casting surface formed by a cylindrical tube of a material selected from the group consisting of copper and copper alloy optionally with a coating thereon and has a plurality of longitudinal water flow passages extending through the tube having a thickness of no more than 80 millimeters, the cylindrical tube capable of changing crown of the casting surface with changes in temperature of water flowing through the passages during casting,
b. a metal delivery system capable of forming a casting pool supported on the casting surfaces of the casting rolls above the nip with side dams adjacent ends of the nip to confine the casting pool,
c. at least one sensor capable of sensing thickness profile of the cast strip downstream of the nip and generating electrical signals indicative of the thickness profile of the cast strip,
d. a water flow controller capable of controlling the temperature of the water flowing through the longitudinal water flow passages in the tube thickness,
e. a casting roll drive system capable of counter rotating the casting rolls and varying the speed of the casting rolls during casting, and
f. a control system responsive to electrical signals received from the sensors capable of controlling the casting roll drive to vary the speed of rotation of the casting rolls and controlling the water flow controller to vary the temperature of the water flow circulated through the water flow passages to control roll crown of the casting rolls during a casting campaign.
Again, the cylindrical tube may have an internal cavity to define the cylindrical tube and provide for flexure thereof as described above. Tube may be between 40 and 80 millimeters in thickness or between 60 and 80 millimeters in thickness.
The longitudinal water flow passages in the tube thickness may be arranged in three pass sets round the cylindrical tube thickness, so that the cooling water circulates through the three passages of the set in series before exiting the casting roll either directly or through the internal cavity. Alternatively, the longitudinal water flow passages in the tube thickness may be arranged in single pass sets round the cylindrical tube thickness so that the cooling water circulates through one passage before exiting the casting roll either directly or through the internal cavity.
At least one sensor capable of sensing thickness profile of the cast strip may be adjacent to pinch rolls through which the strip first passes after casting. A plurality of sensors capable of sensing thickness profile of the cast strip may be positioned laterally across the strip.
Various aspects of the invention will become apparent to those skilled in the art from the following detailed description, drawings and claims.